For the purpose of operating low-pressure gas discharge lamps, increasing use is being made of ballasts which not only start the relevant gas discharge lamp and supply it with the required voltage and the desired current, but also monitor the operation of the lamp. For example, DE 44 10 492 A1 has disclosed an electronic ballast having a freely oscillating inverter, the operation of which in stopped when a maximum lamp voltage is exceeded. For this purpose, there is connected to the terminal of the discharge lamp to which AC voltage is applied a voltage divider whose output is connected via a four-layer diode to the gate of a thyristor. The thyristor is connected to a base terminal of the inventor half bridge and blocks the inventor when it is started. If the voltage present across the gas discharge lamp exceeds a threshold value, the thyristor thereby turns the inventor off. The thyristor is supplied with current from the intermediate circuit voltage via a resistor, and thus is held in the conducting state. An at least brief disconnection of the ballast from the power system is necessary for the purpose of restarting the electronic ballast after a change of lamp. However, automatic restarting is frequently desired, that it to say after a change of lamp the electronic ballast is to be operationally ready again without further measures, and the low-pressure gas discharge lamp is to be supplied with current or voltage.
For this purpose, EP 0 239 793 B1, for example, has disclosed a circuit of a freely oscillating electronic ballast in which the defective behavior, leading to increased lamp voltages, of a gas discharge lamp is detected via the increased voltage drop across a resonance inductor which is connected in series with the lamp. The resonance inductor is coupled to a secondary winding which is connected via a trigger circuit to the gate electrode of a thyristor. In the case of a fault, the latter earths the base of an inventor transistor in order to switch the inventor circuit to be inactive. The thyristor is supplied with a holding current from the intermediate circuit voltage via a resistor combination. After a change of lamp, said current is briefly taken over by a commutation capacitor which is charged up to the intermediate circuit voltage via a resistor by means of a terminal with the lamp taken out. When a lamp is inserted, its filament switches the charged terminal to frame, the capacitor briefly taking over with its other terminal the current flowing through the thyristor, with the result that said thyristor is turned off and the generator circuit can restart.
Said circuit is freely oscillating control circuit.
In order to be able to undertake as precise as possible a setting of the power converted at the lamp, or of the voltage present and of the current flowing, and to avoid reactions of properties of the gas discharge lamp on the operation of the ballast, there is a trend toward externally controlled ballasts which operate at a prescribed frequency. Such a ballast is disclosed, for example, in EP 0 727 921 A2. The electronic ballast contains a generator circuit for generating a lamp AC voltage, it being possible for the generator circuit to be stopped operating via a control input. Connected to the latter is a voltage monitoring unit which turns off the generator circuit when a maximum voltage across the lamp is exceeded. In more concrete terms, an appropriate generator circuit is disclosed in the application details for the L 6569 of SGS Thomson Microelectronics. The generator described there has control terminals for controlling an inventor half bridge, and operates at an operating voltage of, for example, 15 volts. If this operating voltage is lowered below a prescribed threshold value UVLO, the circuit blocks the connected inventor half bridge. The operating voltage can be lowered via a thyristor, which is then further supplied with the holding current from the intermediate circuit voltage.
In order to restart the circuit, the latter must be disconnected from the power system until the holding current has decayed and the thyristor has become free.